When Ally is getting ready for the prom, Elaine does her hair. Before they show Billy, her hair looks great, after Billy shows up it is all messed up and falling out. Then Billy goes away and it is nice again.

This is NOT a blooper. Obviously the person who posted this is not a regular viewer of Ally, or they are not too bright! The whole point of that scene was that even tho she looked lovely, when he walked in she FELT like she looked awful, because she is insecure, and as soon as he left, her opinion of herself went back to normal.
This show is filled with little things like this, some just more obvious than others. It's actually one of the best parts of the show, you have to pay attention and use your brain and eyes to really get all the little jokes they throw in.
Probably all the other comments about this show are similarly erroneous. They do all kinds of strange effects and odd shots thruout the show or it would no longer BE Ally McBeal!
Like when she saw a cute guy and her tongue took on gigantic proportions and dropped to the floor (rivaling even that of Gene Simmons), it meant she was 'panting over' the cute guy, not that they used the wrong tongue!! Get it now?? Enjoy!